This application is based on Japanese Patent Application No. 11-241789 filed Aug. 27, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a control apparatus for an automatic transmission of an automotive vehicle, and more particularly to a downshift of the automatic transmission which is initiated when the input speed of the transmission is lower than the synchronizing speed of an operating position to which the transmission is shifted down.
2. Discussion of the Related Art
There is widely known a power transmitting system for an automotive vehicle, which includes (a) an automatic transmission which is shifted to a selected one of a plurality of operating positions by engaging and releasing actions of a plurality of frictional coupling devices by respective hydraulic cylinders, and (b) a hydraulic control device for supplying a pressurized fluid to one of the hydraulic cylinders to effect an engaging action of a corresponding one of the frictional coupling device, the hydraulic control device including a pressure regulating device for controlling the pressure of the pressurized fluid applied to the above-indicated one hydraulic cylinder. JP-A-5-312261 discloses an example of such a power transmitting system, which is adapted to shift down the automatic transmission from a high-gear position to a low-gear position by releasing a frictional coupling device which has been engaged to establish the high-gear position, while at the same time engaging a frictional coupling device for establishing the low-gear position, that is, adapted to effect a so-called xe2x80x9cclutch-to-clutch shifting actionxe2x80x9d wherein the hydraulic pressure of the frictional coupling device which has been engaged to establish the high-gear position is feedback-controlled such that the input speed of the automatic transmission changes at a predetermined rate while the hydraulic pressure of the frictional coupling device for establishing the low-gear position is held at a standby level, and such that the frictional coupling device for the high-gear position is fully released while the frictional coupling device for the low-gear position is fully engaged, after the input speed has substantially reached the synchronizing speed of the low-gear position.
In the conventional clutch-to-clutch shifting action described above, the hydraulic pressures of the two frictional coupling devices must be concurrently controlled, requiring a difficult control of the hydraulic control device. Where a downshift command to shift down the automatic transmission to the low-gear position is generated a comparatively short time after the generation of an upshift command to shift up the automatic transmission to the high-gear position, the clutch-to-clutch downshift cannot be achieved in an adequate manner unless the piston of the hydraulic cylinder (piston of an accumulator communicating with the hydraulic cylinder, if the accumulator is provided) has already reached the stroke end on the engaging side. In this case, therefore, the downshift to the low-gear position must be initiated only after the high-gear position has been once established. This arrangement requires a relatively long time to complete the downshift of the automatic transmission.
It is accordingly an object of the present invention to provide a control apparatus for an automatic transmission of an automotive vehicle, which is adapted to initiate a downshift of the automatic transmission from a high-gear position to a low-gear position while the input speed of the automatic transmission is lower than the synchronizing speed of the low-gear position, and which permits the downshift to be smoothly effected, by controlling an engaging action of only a frictional coupling device for establishing the low-gear position.
The above object may be achieved according to the principle of the present invention, which provides an apparatus for controlling an automatic transmission of an automotive vehicle, which is shifted to a selected one of a plurality of operating positions by an engaging action of a frictional coupling device, the apparatus comprising an overshoot engagement control device which is operated to shift the automatic transmission to the selected one operating position such that a shifting action of the automatic transmission is initiated while an input speed of the automatic transmission is lower than a synchronizing speed of the selected one operating position. The overshoot engagement control device controls an engaging force of the frictional coupling device such that the input speed is first raised above the synchronizing speed and is then lowered down toward the synchronizing speed.
In the vehicle transmission control apparatus of the present invention constructed as described above, the shifting action of the automatic transmission to a selected one of the operating positions is initiated while the input speed of the automatic transmission is lower than the synchronizing speed of the selected operating position. To shift the automatic transmission to the selected operating position, the engaging force of the frictional coupling device is controlled such that the input speed is first raised above the synchronizing speed and is then lowered down at a low rate toward the synchronizing speed. In this control arrangement, the engaging force of the frictional coupling device while the input shaft speed is higher than the synchronizing speed can be controlled in a manner similar to that used for controlling the frictional coupling device to shift up the automatic transmission to the selected operating position.
Thus, the automatic transmission can be shifted down to the selected operating position by controlling the engaging action of the single frictional coupling device, so that when the automatic transmission is commanded to be shifted down to the selected operating position a comparatively short time after the automatic transmission was commanded to be shifted up to the operating position one step higher than the selected operating position, the automatic transmission can be immediately shifted down to the selected operating position, without once establishing the higher operating position.
The automatic transmission to which the control apparatus of the present invention is applicable may include a plurality of planetary gear sets, and a plurality of hydraulically operated frictional coupling devices such as clutches and brakes for connecting rotary elements of the planetary gear sets to each other or fixing those rotary elements to the housing of the automatic transmission. In this case, the selected one of the operating positions of the automatic transmission which have respective different speed ratios is established by engaging and releasing actions of the appropriate frictional coupling devices. The automatic transmission may be automatically shifted down or up in response to a shifting command which is generated on the basis of the vehicle running speed and the operating amount of the accelerator pedal. Alternatively, the automatic transmission is commanded to be shifted in response to an operation of a shift lever or switch by the vehicle operator.
The automotive vehicle equipped with the present transmission control apparatus may use a drive power source selected from among various drive power sources such as an engine operated by combustion of a fuel, and an electric motor operated with an electric energy. The output of the drive power source may be electrically controlled depending upon the operating amount of the accelerator pedal or other accelerating member provided for accelerating the vehicle, or the drive power source may be mechanically connected to the accelerating member so that the output of the drive power source is directly controlled by the accelerating member.
The shifting action of the automatic transmission initiated while the input speed is lower than the synchronizing speed of the selected operating position includes not only a downshift from the higher-gear position to the selected operating position, but also a shifting action which involves initial releasing of the frictional coupling device and consequent reduction of the input speed of the automatic transmission when the accelerating member which has been operated is released to the non-operated position during running of the vehicle with the automatic transmission placed in the selected operating position, and subsequent engagement of the frictional coupling device when the accelerating member is operated. The latter shifting action will be described later with respect to a fifth preferred form of the invention. When the automatic transmission is shifted sown from the higher-gear position to the selected operating position, the selected operating position can be established according to the present invention by simply controlling only the engaging action of the corresponding frictional coupling device. Namely, the frictional coupling device which has been engaged to establish the higher-gear position may be released immediately after the generation of a command to effect the downshift to the selected operating position.
The engaging force of the frictional coupling device to effect the downshift to the selected operating position can be controlled in the same manner as used to effect the upshift to the selected operating position. However, the engaging force to effect the downshift may be controlled in a manner totally different from that used to effect the upshift.
In a first preferred form of the present invention, the automatic transmission includes a hydraulic cylinder operated to effect the engaging action of the frictional coupling device, and the apparatus further comprises a pressure regulating device for controlling a hydraulic pressure of the hydraulic cylinder. In this form of the invention, the overshoot engagement control device controls the pressure regulating device, to effect a feed-forward control of the hydraulic pressure of the hydraulic cylinder, and the overshoot engagement control device includes: time calculating means for calculating a first time required for the input speed of the automatic transmission to reach the synchronizing speed of the selected one operating position; a time data memory device which stores a second time required for a piston of the hydraulic cylinder to reach an engaging stroke end thereof; and timing determining means for comparing the first time with the second time, and thereby determining a point of time at which a supply of a pressurized fluid to the hydraulic cylinder is initiated, such that the piston has reached the engaging stroke end to initiate the generation of the engaging force of the frictional coupling device, when the input speed of the automatic transmission has substantially reached or exceeded the synchronizing speed.
In the present first preferred form of the invention, the frictional coupling device to establish the selected operating position is engaged by activation of the corresponding hydraulic cylinder, and the overshoot engagement control device controls the pressure regulating device to effect the feed-forward control of the hydraulic pressure of the hydraulic cylinder. The timing determining means of the overshoot engagement control device is basically adapted to determine the point of time at which the supply of the pressurized fluid to the hydraulic cylinder is initiated, such that the supply is initiated when the first time required for the input speed to reach the synchronizing speed, as calculated by the time calculating means, has become equal to the second time required for the piston of the hydraulic cylinder to reach the engaging stroke end, which second time is stored in the time data memory. By initiating the supply of the pressurized fluid to the hydraulic cylinder at the thus determined point of time, the piston of the hydraulic cylinder has reached the engaging stroke end to initiate the generation of the engaging force of the frictional coupling device, when the input speed of the automatic transmission has substantially reached or exceeded the synchronizing speed. In this preferred form of the invention, the overshoot engagement of the frictional coupling device can be always achieved at the appropriate timing, so as to minimize a shifting shock of the automatic transmission, which would take place due to a variation in the point of time at which the generation of the engaging force of the frictional coupling device is initiated.
The hydraulic cylinder used for engaging the frictional coupling device in the first preferred form of the invention may be replaced by other types of frictional coupling device such as an electromagnetic clutch which is engaged by an electromagnetic force. In the first preferred form of the invention using the hydraulic cylinder, the feed-forward control of the hydraulic pressure of the hydraulic cylinder is desirable. However, the hydraulic pressure may be controlled such that the piston of the hydraulic cylinder is held at a predetermined standby level slightly lower than the synchronizing speed, until the input speed of the automatic transmission has reached the synchronizing speed, and such that the hydraulic pressure is raised to fully engage the frictional coupling device.
In one advantageous arrangement of the first preferred form of the invention, the time calculating means detects a rate of change of the input speed, obtains the synchronizing speed on the basis of a speed ratio of the selected one operating position and an output speed of the automatic transmission, and calculates the first time by dividing a difference between the obtained synchronizing speed and the input speed by said rate of change of said input speed.
The output speed of the automatic transmission, which represents the vehicle running speed, may be considered substantially constant. However, the synchronizing speed may be obtained with higher accuracy, if the output speed and a rate of change of the output speed are used in addition to the input speed and the rate of change of the input speed. This manner of obtaining the synchronizing speed is desirable particularly where the frictional coupling device is engaged to effect a downshift of the automatic transmission to the selected operating position during coasting of the vehicle without an operation of the accelerating member. The input and output speeds of the automatic transmission may be replaced by the rotating speeds of other appropriate rotary members of the automatic transmission.
The second time required for the piston of the hydraulic cylinder to reach the engaging stroke end differs depending upon the hydraulic pressure in the hydraulic control circuit. Usually, this hydraulic pressure is adjusted according to the output of the vehicle drive source, for instance, the opening angle of the throttle valve of the engine. In this respect, it is preferred that the second time be determined or adjusted depending upon the output of the vehicle drive force (e.g, opening angle of the throttle valve). The second time may be determined on the basis of other parameters such as the temperature of the working fluid, which influence the second time. To eliminate a difference in the characteristic between the individual hydraulic cylinders, the second time may be determined by actually detecting the time required for the piston to reach the engaging stroke end. In this case, the second time stored in the time data memory is updated by learning compensation based on the detected time.
The timing determining means of the overshoot engagement control device is basically adapted to initiate the supply of the pressurized fluid to the frictional coupling device when the first time has become substantially equal to the second time. To prevent a torque from being transmitted to the engine through the automatic transmission, the point of time at which the supply is initiated may be purposely delayed so that the generation of the engaging force by the frictional coupling device is initiated only after the input speed of the automatic transmission has exceeded the synchronizing speed. The point of time at which the generation of the engaging force is initiated may be adjusted by controlling the pressure of the pressurized fluid or a pattern of change of the pressure.
In a second preferred form of this invention, the automatic transmission includes a hydraulic cylinder operated to effect the engaging action of the frictional coupling device, and the apparatus further comprises: manual shifting engagement control means operated in response to a manual operation to effect a manual downshift of the automatic transmission to the selected one operating position while the vehicle is running in a power-off state in which a drive force is not transmitted through the automatic transmission to drive wheels of the vehicle, the manual shifting engagement control means controlling a hydraulic pressure of the hydraulic cylinder in a manner different from that of the overshoot engagement control device; and transition releasing means operated in response to an increase in an amount of operation of an accelerating member for accelerating the vehicle, in the process of the manual downshift of the automatic transmission, for first draining the hydraulic cylinder to release the frictional coupling device, and then enabling the overshoot engagement control device to operate to control the engaging force of the selected one frictional coupling device.
The term xe2x80x9cpower-off statexe2x80x9d of the vehicle is interpreted to means a state in which the drive force is not transmitted from a vehicle drive source such as an engine to the vehicle drive wheels through the automatic transmission. This xe2x80x9cpower-off statexe2x80x9d includes not only a state in which the operator""s required output of the drive power source is zero, namely, a state in which the accelerator pedal or other accelerating member for accelerating the vehicle is in the non-operated position, but also a state in which the vehicle drive force is not transmitted to the drive wheels because the operating amount of the accelerating member is smaller with respect to the running speed of the vehicle. The term xe2x80x9can increase in an amount of operation of an accelerating memberxe2x80x9d is interpreted to mean an operation of the accelerating member by the vehicle operator to increase the output of the vehicle drive source.
In the second preferred form of this invention, the manual shifting engagement control means is operated in response to a manual operation to effect a manual downshift of the automatic transmission to the selected operating position while the vehicle is in the power-off state, so that the hydraulic pressure of the frictional coupling device is controlled in a manner different from that of the overshoot engagement control device. When the accelerating member is operated to increase the output of the vehicle drive power source in the process of the manual downshift, the transition releasing means is operated to first drain the hydraulic cylinder of the frictional coupling device to release the frictional coupling device, and then enable the overshoot engagement control device to operate to effect the overshoot engagement of the frictional coupling device. In this control arrangement, the residual pressure in the hydraulic cylinder in the engagement control of the frictional coupling device by the manual shifting engagement control means will not influence the overshoot engagement control of the frictional coupling device, since the hydraulic cylinder is first drained to release the frictional coupling device before the overshoot engagement control is initiated. Accordingly, the engaging force of the frictional coupling device can be adequately controlled so that the piston of the hydraulic cylinder has reached the engaging stroke end to initiate the generation of the engaging force when the input speed of the automatic transmission has substantially reached or exceeded the synchronizing speed.
The manual shifting engagement control means is arranged to control the hydraulic pressure of the hydraulic cylinder, so as to smoothly increase an engine braking force to be applied to the vehicle in the manual downshift, on an assumption that the manual downshift is selected by the vehicle operator in the power-off state, in order to apply the engine brake to the vehicle. While the overshoot engagement control device according to the first preferred form of the invention described above is arranged to control the hydraulic pressure while taking into account the first time required for the input speed to reach the synchronizing speed, the manual shifting engagement control means according to the present second preferred form of the invention may be arranged to effect the overshoot engagement control of the frictional coupling device without taking into account the above-indicated first time (synchronizing time). The present second preferred form relates to the control of the frictional coupling device when the operating amount of the accelerating member is increased in the process of the manual downshift in the power-off state, that is, when the vehicle state is changed from the power-off state to the power-on state. The present control to first drain the hydraulic cylinder to release the frictional coupling device and then effect the overshoot engagement control of the frictional coupling device is desirable where the automatic transmission is commanded to be shifted down to the selected position in the process of the upshift from the selected position to the higher-gear position. The present control is also desirable in the sixth preferred form of the invention described later, where the operating amount of the accelerating member is increased in the process of releasing of the frictional coupling device by the power-off releasing means.
The manual shifting engagement control means may be operated only when the manual downshift of the automatic transmission is selected by a manual operation by the vehicle operator while the accelerating member is in the non-operated position. However, the manual shifting engagement control means may be operated when the manual downshift is selected while the accelerating member is in an operated position, as long as the power-off state (in which a vehicle drive force is not transmitted from the vehicle drive power source to the vehicle drive wheels through the automatic transmission) is detected, for example, on the basis of the rotating speeds of the input and output members of a torque converter connected to the automatic transmission.
In a third preferred form of this invention, the overshoot engagement control device controls the engaging force of the frictional coupling device when the automatic transmission is commanded to be shifted down to the selected one operating position during coasting of the vehicle while an accelerating member for accelerating the vehicle is placed in a non-operated position, such that the input speed is first raised above the synchronizing speed and is then lowered down toward the synchronizing speed, and such that the engaging force is not generated by the frictional coupling device while the input speed is not higher than the synchronizing speed.
The term xe2x80x9ccoastingxe2x80x9d of the vehicle is interpreted to mean a running of the vehicle with the accelerating member placed in the non-operated position, and the term xe2x80x9ccommanded to be shifted down to the selected one operating position during coastingxe2x80x9d is interpreted to mean a downshift of the automatic transmission to the lower-gear position having the higher speed ratio, in response to a reduction of the vehicle running speed without an operation of the accelerating member. Usually, the downshift during coasting of the vehicle does not involve an engine brake application to the vehicle. In the present third preferred form of the invention, the overshoot engagement control means is arranged to effect the overshoot engagement of the frictional coupling device such that the input speed is first raised above the synchronizing speed and is then lowered down toward the synchronizing speed, as described above with respect to the apparatus constructed according to the principle of the invention.
In the third preferred form of the invention, the overshoot engagement of the frictional coupling device is achieved upon generation of a command to effect a downshift of the automatic transmission to the selected operating position during coasting of the vehicle. The overshoot engagement control device controls the engaging force of the frictional coupling device such that the engaging force is not generated by the frictional coupling device while the input speed of the automatic transmission is not higher than the synchronizing speed, so that the drive force is not transmitted from the drive wheels to the vehicle drive power source through the automatic transmission, namely, an engine brake is not applied to the vehicle, when the automatic transmission is shifted down to the selected operating position.
In this third preferred form of the invention, too, the moment of generation of the engaging force of the frictional coupling device may be adjusted by suitably determining the point of time at which the supply of the pressurized fluid to the frictional coupling device is initiated.
In a fourth preferred form of this invention, the apparatus further comprises means for holding the frictional coupling device in its released state while an accelerating member for accelerating the vehicle is placed in a non-operated position in the process of a downshift of the automatic transmission to the selected one operating position during coasting of the vehicle, and wherein the overshoot engagement control device is operated, when the accelerating member is operated, to control the engaging force of the frictional coupling device such that the input speed is first raised above the synchronizing speed and is then lowered down to the synchronizing speed.
The phrase xe2x80x9cwhen the accelerating member is operatedxe2x80x9d means, for example, an operation of an accelerator pedal by the vehicle operator to increase the output of the vehicle drive power source. The phrase xe2x80x9cwhile an accelerating member for accelerating the vehicle is placed in a non-operated positionxe2x80x9d means that the accelerating member is not in operation, in the presence of the vehicle operator""s desire to accelerate the vehicle.
In the fourth preferred form of the invention, the frictional coupling device is held in the released state while the accelerating member is not in operation in the process of a downshift of the automatic transmission to the selected operating position during coasting of the vehicle. This control arrangement prevents the application of an engine brake to the vehicle, namely, prevents the automatic transmission to transmit a negative torque from the drive wheels to the vehicle drive power source. Further, when the accelerating member is operated, the overshoot engagement control device controls the engaging force of the frictional coupling torque, so that the vehicle drive force can be generated in a short time after the operation of the accelerating member.
The fourth preferred form of the invention may be adapted such that the frictional coupling device is engaged after the input speed has exceeded the synchronizing speed, while the accelerating member is held in the non-operated position in the process of the coasting downshift to the selected operating position. However, the frictional coupling device may be held in the released state even after the input shaft has exceeded the synchronizing speed. Then, the frictional coupling device may be controlled according to the following fifth preferred embodiment of the invention.
In a fifth preferred form of this invention, the apparatus further comprises power-off releasing means for releasing the frictional coupling device when the vehicle is brought into a power-off state in which a drive force is not transmitted through the automatic transmission to drive wheels of the vehicle, during running of the vehicle while the automatic transmission is placed in the selected one operating position with the frictional coupling device placed in an engaged state thereof, and wherein the overshoot engagement control device is operated in response to an increase in an amount of operation of the accelerating member operated while the frictional coupling device is held in a released state thereof by the power-off releasing means, to control the engaging force of the frictional coupling device such that the input speed is first raised above the synchronizing speed and is then lowered down to the synchronizing speed.
In the fifth preferred form of the invention, the frictional coupling device is released by the power-off releasing means when the vehicle is brought into the power-off state during running of the vehicle while the automatic transmission is placed in the selected operating position with the frictional coupling device placed in the engaged state. In the absence of a one-way clutch to establish the selected operating position, this arrangement is effective to prevent the application of an engine brake to the vehicle when the vehicle is brought into the power-off state. When the operating amount of the accelerating member is increased while the frictional coupling device is held in the released state by the power-off releasing means, the frictional coupling device is engaged by the overshoot engagement control means, so that the drive force can be produced in a short time after the operating amount of the accelerating member has been increased. Thus, the frictional coupling device can be adequately released and engaged depending upon when the vehicle is placed in the power-off state or power-on state, and the drive force can be more suitably produced by overshoot engagement of the frictional coupling device, than where the one-way clutch is used to prevent the engine brake application.
The power-off releasing means may be operated only when the vehicle is placed in the power-off state with the accelerating member being released to the non-operated position. However, the power-off releasing means may be operated while the accelerating member is in an operated position, as long as the power-off state is detected, for example, on the basis of the rotating speeds of the input and output members of a torque converter connected to the automatic transmission.